Background: Ipilimumab (Ipi) is a humanized CTLA-4 antibody that blocks binding of CTLA-4 to B7, permitting T cell activation through CD28. Phased in Ipi added to chemotherapy (C) may enhance efficacy in NSCLC. Methods: Patients with stage 2 or 3A NSCLC received neoadjuvant carboplatin AUC6 plus paclitaxel 200 mg/m² every 21 days for 3 cycles and Ipi (10 mg/kg) was given on day 1 for cycles 2 and 3. Blood for immune profiling of circulating T cells was collected at baseline, after chemotherapy alone, and after chemotherapy plus Ipi. Tumor infiltrating lymphocytes (TIL) were derived from 7 available tumors. Polychromatic flow cytometry (PFC) analyses were performed on peripheral blood mononuclear cells (PBMC) and TIL. Objective response rates were assessed according to RECIST 1.1 criteria. Results: Of the 24 patients enrolled on this study, objective responses after 3 cycles of neoadjuvant C plus ipi included 2 PD, 8 SD, and 14 PR. Phenotypic analyses revealed that PBMC from all 24 patients were highly activated following two cycles of Ipi (cycle 3) as evidenced by significantly increased frequencies of CD28, ICOS, HLA-DR, PD-1, and CTLA-4 expressing CD4+ cells; and ICOS, HLA-DR, and CTLA-4 expressing CD8+ cells. The frequencies of Tregs were highly variable among the 24 participants. Two of the 24 participants had levels of MDSC cells above 15%. TIL contained far greater frequencies of activated CD4+ and CD8+ cells than found in the PBMC at cycle 3. Tumor associated antigen (TAA)-specific CD4+ or CD8+ cells were detected at baseline in 4 patients (24%), but their relative frequencies remained unaltered by Ipi therapy. No patients developed detectable de novo TAA reactivities while on Ipi therapy. Conclusions: Combined neoadjuvant Ipi plus chemotherapy produced significantly increased frequencies of highly activated CD4+ and CD8+ populations in the peripheral blood and the tumor microenvironment. TAA-specific CD4+ or CD8+ cells were detected in PBMC at baseline in a subset of patients. No TAA-reactive T cells were detected among the 7 TIL samples analyzed. Analysis for predictive or pharmacodynamic biomarkers is ongoing. Clinical trial information: NCT01820754